Stardust Memories

To see a World in a Grain of Sand,
And a Heaven in a . . . Meteorite, . . . .

Admirers of William Blake will recognize (and surely not forgive) the dissonant change to the first sentence from the 18th-century poet's work Auguries of Innocence. The rocky substitution doesn't slip off the tongue as readily as Blake's "Wild Flower," but it may have literal truth. In the past few years, a small group of scientists has been sifting through meteorite fragments, searching for dust grains that were formed around other stars—before the formation of our own solar system. These presolar grains are indeed from a heaven, one much older and in a different place from the one we see in the night sky.

The scientists who study these grains of ancient stardust are a breed apart from the astronomers who rely on telescopes to gather information about the stars. Calling themselves "cosmochemists," these scientists bring stardust into the laboratory, where they examine the structure and composition of the dust grains with electron microscopy, x-ray diffraction and ion microprobes (which permit the measurement of various isotopes in the grains). These laboratory surveys have turned up an assortment of minuscule grains (usually less than a few micrometers in diameter) whose "peculiar" isotopic composition suggests that they could not have been created by the processes in and around our sun. They could only have formed around other types of stars and then made their way into the presolar nebula that became our solar system.

Roy Lewis and his colleagues at the University of Chicago were among the first to isolate and identify a type of presolar stardust back in 1987. It turned out to be a crystal form of carbon—diamond—each about 20 angstroms across and consisting of only about 1,000 atoms. "Diamond stardust came as quite a surprise," said Lewis during a recent telephone interview. Since this first discovery, cosmochemists have come to appreciate that only the sturdiest of substances can survive the laboratory techniques used to separate the presolar grains from the rest of the meteorite. The process is so severe that Lewis's colleague Edward Anders (also at Chicago) has likened it to "burning down the haystack to get at the needle."

Diamond, it turns out, is the most common type of presolar grain found. But several other mineral forms have also been discovered, including silicon carbide, graphite, corundum (aluminum oxide), silicon nitride and trace amounts of titanium carbide, zirconium carbide and molybdenum carbide.

Once the grains have been isolated and identified—"tedious work, requiring a great deal of care," says Lewis—the next step is to deduce the type of star that could synthesize the various isotopes in the grain and produce the grain itself. This is where the hands-on cosmochemists must turn to the theorists, whose pencil-and-paper calculations of stellar nucleosynthesis lie at the heart of modern astrophysics. The theorists' calculations have shown that many of the presolar grains were made by stars at or near the end of their lives. In particular, catastrophic events such as supernova explosions and the dusty exhalations of certain red-giant stars during their death throes seem to have been major contributors to the heavy elements and the exotic isotopes found in the presolar grains.

However, "certain observations can't be so readily explained by the standard stellar models," according to Lewis. For example, cosmochemists have now studied nearly 20,000 grains of presolar silicon carbide. The majority of these grains have an excess of two heavy silicon isotopes (29Si and 30Si) compared to the lighter isotope 28Si. Stellar models suggest that an excess of 29Si and 30Si could be produced in the outer layers of carbon-rich red-giant stars, in a well-defined ratio. If one plots the relative ratios of 29Si/28Si to 30Si /28Si for each grain, a straight line is formed, dubbed the "mainstream." The problem, according to Lewis, is that "theory predicts the enrichments in 29Si and 30Si should spread the grains out on a correlation with a slope between 0.3 and 0.5, not the observed slope of 1.34." In other words, the mainstream grains contain more excess 29Si relative to the excess 30Si than theory would suggest. Some astronomers have proposed ad hoc explanations invoking notions of galactic chemical evolution, "but this remains a little hard for many of us to immediately accept," says Lewis.

Isotopic abundances of other elements in these presolar grains also suggest additional problems with some of the stellar models. In only its first decade, the new field of laboratory cosmochemistry is now challenging some fundamental aspects of stellar physics. "Unfortunately," says Lewis, "many of the astrophysicists are not paying much attention to our work." Perhaps they should. Where else does science rise to the level of a William Blake poem?

For those who may have forgotten, the first sentence from Blake's poem ends:

Hold Infinity in the palm of your hand,
And Eternity in an hour.

Scientists have yet to hold infinity in their hands, but they've held stuff older than our solar system, and even the poet would have been impressed by that.—Michael Szpir